Process for the separation of hydrocarbons



P 1 1937- c. H. GREENEWALT 2,076,557

PROCESS FOR THE SEPARATION OF HYDROCARBONS Filed July 5, 1935 30 f/YPANS/O/V VALVf a CALANDR PUMP C rawford Hfireene NTOR' ATTORNEY.

Patented Apr. 13, 1937 UNITED STATES PATENT OFFICE PROCESS FOR THE SEPARATION OF HYDROCARBONS Crawford H. Greenewalt signor to E. I. du Pont Wilmington, Del., a corporation of Wilmington, de Nemours & Company,

DeL, a-

Delaware Application July 5, 1935, Serial No. 30,029

2 Claims.

from complex hydrocarbon mixtures.

One object of this invention is to provide an eflicient process whereby a mixture of hydrocarbon gases is separated into its component parts. Another object is the recovery of propylene from hydrocarbon mixtures which also contain ethane, propane, butylene, butane, etc., but is substantially free from hydrocarbons boiling below ethane. Afurther object is the recovery of ethane as a fraction from the aforesaid hydrocarbons. A still further object is the accomplishment of the above ends with a minimum of equipment and of operating steps. Other objects will appear hereinafter.

These objects are accomplished by the following invention which comprises subjecting the hydrocarbon mixture, from which such low-boiling materials as hydrogen, methane and ethylene have already been removed, to a single fractional distillation under conditions of temperature and pressure which permit the separation of most of the components boiling lower than propylene as a gas, and the removal of a propylene rich fraction and of a fraction containing principally butylene as separate liquid products.

The greatest source of such olefins as ethylene, propylene, and butylene is the gaseous products which result from the vapor phase cracking of petroleum. Such cracked petroleum gases often contain hydrogen, methane, ethylene, ethane, propylene, propane, butylene, butane, etc. My invention is especially applicable to the recovery of propylene from the hydrocarbon mixtures which remain after the crude cracking gases have been processed for the recovery of ethylene. Such an ethylene separation process, an example of which has been described in my copending application Serial No. 30,030, filed July 5, 1935, usually removes hydrogen, methane, and ethylene from the gas mixture. My invention, therefore, is especially advantageous when operated in conjunction with a process for ethylene recovery if it is desired to extend the separation process to include the isolation of propylene and of butylene.

The accompanying drawing represents diagrammatically a complete apparatus for the separation of the components of the aforesaid mixture. Reference will be made to this drawing in order to properly describe the invention.

The following represents a preferred embodiment of the process and apparatus. It is to be understood that the details of this apparatus,

column. The liquid fraction which are well known to those skilled in the art, have been omitted for sake of clarity, and that modifications in details of construction and in arrangement of parts may be made without departing from the spirit of my invention.

The hydrocarbon mixture which may contain ethane, propylene, propane, and butylene, if it is 1 available in the liquid stage, is pumped by means of pump 2 to a pressure of approximately 22 atmospheres and forced'through pipe 4 and section 6 of heat exchanger 8 and thus through pipe [0 into the rectifying column I2. The liquid bydrocarbon mixture is therefore preheated by the heat exchanger 8. If, however, the hydrocarbon mixture is available in the vapor form, it is compressed to about 22 atmospheres and cooled by apparatus not shown and the resulting gas-liquid mixture introduced directly into. the rectifying of the hydrocarbon mixture introduced into the rectifying column I2 collects in the calandria It at the base of the column and is boiled with steam, passing through coil l6. This liquid in the calandria is boiled to such an extent that there is a complete expulsion of hydrocarbons boiling below butylene. The liq- 25 uid bottoms in calandria l4 consisting of principally butylene and higher boiling hydrocarbons are withdrawn by way of pipe l8 and pass through section 20 of heat exchanger 8 and thus are withdrawn from the system by way of pipe 22. Condenser 24 at the top'of column I2 is cooled with water which serves to condense the greater part of the constituents boiling above ethane and thus returns them to the column as a reflux. The ethane vapor, most of which remains uncondensed, together with a small amount of propylene and propane, emerges from the top of condenser 24 through pipe 26 and is expanded by means of a throttling valve 28 into pipe 30 which carries the ethane into other parts of the separation system. The column pressure, which is maintained at about 22 atmospheres, is controlled by release of non-condensed vapors through the above mentioned expansion valve 28. A'liquid fraction is withdrawn by way of pipe 32 from the liquid space on one of the plates just below the column condenser 24. This liquid contains propylene, propane and a small amount of ethane, but is free from contaminating olefins such as butylene.

The gaseous products of most cracking operations contain both propylene and propane. These two hydrocarbons have boiling points differing by only 3 C. No attempt at separation is made in the process just described, and both of the above materials are recovered as a -3 fraction.

It is, of course, well known that the gases from vapor phase cracking processes contain a comparatively small percentage of propane in comparison with propylene. My invention is especially suitable for handling such gases and yields a product sufllciently high in propylene to meet most industrial requirements.

The operating pressure of the fractionation l0 process described above may be varied within certain limits without adverse effect. It is preferred, however, to conduct the fractionation at a pressure of 20-25 atmospheres, since water can satisfactorily be used for cooling purposes within this range. At pressures much below atmospheres, for example, 15 atmospheres, the losses of propylene in the vent gas emerging from the top of the columnare excessive unless refrigeration is supplied to the condenser at a lower temperature level than can be obtained with ordinary cooling water. The use of higher pressures is unnecessary, results in increased power expenditures, and tends to increase the ethane content of the propylene rich product.

One of the principal advantages afforded by my invention is the recovery in a single operation of a producthigh in propylene but free from other oleflns. The achievement of the above result in a single rectifying column permits a ma- 30 terial conservation in equipment cost and simplifies operation and control. It is the usual practice to use two columns for separating propylene from cracked gas mixtures similar to that described in the above example. The first serves 35 for ethane removal, and the second for fractionation of the propylene-propane mixture from the higher boiling hydrocarbons. The use of a separate rectification stage using low temperature refrigeration for separating a propylene-free ethane fraction involves a much greater expenditure of power than does my invention in which a single rectifying column is employed and water is used as the cooling medium. My invention may not effect the complete recovery of propyl- 45 ene, since a small portion of that present in the feed to the rectifying column may be lost in the vent gas.' This lower yield of propylene is, however, more than compensated by the savings in power which result from the elimination 50 of the expensive ethane fractionation step.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to 55 the speciflc embodiments thereof except as defined in the appended patent claims.

I claim:

1. The process of separating the constituents of a fluid mixture comprising ethane, propylene, propane, butylene, and higher boiling hydrocarbons, which comprises compressing said mixture to a pressure of about 20 to 25 atmospheres and rectifying said fluid mixture at this pressure by a counter-current flow of reflux liquors condensed at the top of a rectifying column and by boiling the liquid bottoms at the base of the rectifying column at a temperature sufficient to remove substantially all of the propane and the lower boiling constituents, said reflux liquors being condensed at a temperature below the boiling point of propylene so as to condense substantially all the propylene from the gaseous fraction prior to removal of said gaseous fraction from the system, removing the gaseous fraction comprising essentially ethane, removing some of the reflux liquors just after condensation, said liquor comprising essentially propylene and propane, and removing the liquid bottoms from the base of the rectifying column, said bottoms comprising essentially butylene and the higher boiling hydrocarbons.

2. The process of separating by fractionation the constituents of a liquid mixture comprising ethane, propylene, propane, butylene, and the higher boiling hydrocarbons, which comprises compressing said mixture to about 22 atmospheres, preheating said compressed mixture by passing same in heat exchange relationship with the liquid bottoms obtained from the fractionation system, rectifying this preheated fluid by a counter-current flow of reflux liquors condensed at the top of a rectifying column and by boiling the liquid bottoms at the base of the rectifying column at a temperature suflicient to remove substantially all of the propane and the lower boiling constituents, said reflux liquors being condensed at a temperature below the boiling point of propylene so as to condense substantially all the propylene from the gaseous fraction prior to removal of said gaseous fraction from the system, removing the gaseous fraction comprising essentially ethane, recovering some of the reflux liquor immediately after condensation, said liquor comprising essentially propylene and propane, removing the liquid bottoms comprising essentially butylene and'the higher boiling hydrocarbons and passing same in heat exchange relationship with the incoming fluid mixture so as to preheat same, and then passing said liquid bottoms out of the system.

' CRAWFORD H. GREENEWALT. 

